Outside ads only pay (support the forum) if they are clicked. If they are hidden for a large segment of viewers, then their benefit will be limited. The most problematic ads in my experience are those that attempt to load a slide show of changing pictures. These often cause "script overload" for the browser. So I would suggest that all ads be static (a single display) for a single display instance.
Now, it may be that guest traffic is sufficient to make them pay, but the money is all about click-throughs.
If you can make a buck on advertising, then you'd be doing better than most of today's print media.
Bob
This is some highly beneficial information for making a decision.
I vote you do what's best for yourself and your business and I'll back you 50% or better. I can work around whatever you decide, I'm already paying for my WLT purchases with your credit card.